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With helicopters, is normal landing or auto-rotation landing any different when landing on water?
I noticed something from investigating my other answer to a not-really-related question.
A wiki article here says:

The "water bird" emergency landing is a technique developed by the Canadian Forces to safely land the Sikorsky CH-124 Sea King helicopter when one engine fails while flying over water. The emergency landing technique allows the boat-hull equipped aircraft to land on the water in a controlled fashion.[37]

Which leads me to the obvious question: Is landing any different for water landings? If so, how?
I don't know much about helicopters. For a normal landing, I assume they just hover down and I thought the same thing would be done for a water landing. But the wiki article I read implies that something is different.
For auto-rotation without any engine power, apparently there is a flare at the end. Is this flare done any differently when landing on water?
Basically, the spirit of the question is, how is landing different (if at all) on the water, whether powered or not?

"or emergency descent in general" what do you mean by this?  A controlled, but rapid descent with power? The quickest way of getting a helicopter down with power is to autorotate to about 20 feet then bring the power back in to land normally.

The Sea King is a multi-engine helicopter, this procedure you posted is for a single engine water landing, so its not really an auto-rotation situation, the helicopter still has (reduced) power available, enough probably to slow the descent enough to make water landing without damaging the airframe.

@Simon "emergency descent in general", any landing that needs to take place immediately. Could be powered or not depending on failures. The article seems to distinguish descents over land from descents over water, which I found really odd.

@RonBeyer I also included "emergency descent" in my question. The way I read the article, it seems to distinguish descents over land from descents over water, which I found really odd. Maybe I am not reading it right?

I think you need another edit, perhaps to remove the link to the "water bird" landing altogether. A water bird landing is not an auto-rotation or an "emergency descent".  It is a single engine landing on water from forward flight in a particular type, the Sea King.  What exactly are you asking?  I've voted to close as unclear but will happily retract that if you can edit the question to be precise.  If the question is simply "what is the difference between landing on water and landing on the ground?", then you should ask just that.

As a helicopter pilot, I've never made a water based landing, so I'm not speaking directly from experience, only from my training, but there are a couple things that are missed in the discussion here that I would certainly worry about.
If I had a real engine failure, and was forced to perform an auto-rotation, in all likelihood, I would not end the auto-rotation with zero forward velocity, I would probably perform a running landing (where you slide on your skids). This, of course, is assuming I'm landing on a flat surface where I'm not likely to roll over (such as a street, parking lot, or runway). Holding the flare to bring the helicopter to a complete standstill has a few risks for me, the biggest one being that if I flare and stop my descent while I still have forward velocity, holding the flare will start to increase my altitude again. Higher inertia (heavier) rotors in small helicopters will suffer from this problem, for example, in training, a Robinson R-22 will generally not have this issue, but an R-44 has much heavier rotors, and we get trained to anticipate a flare at the end of the auto-rotation. Depending on the helicopter, weather, and how I've performed my auto-rotation, if I'm in a real emergency, I would rather put the helicopter down on the ground and slide a little bit than suffer a rotor stall at 50 feet, but a running landing is clearly not an option in a water based landing.
Once I've performed an auto, and the helicopter is on the ground, I'd apply the rotor brake to get the rotors to stop moving before I exited the aircraft, and generally speaking, won't have to worry about the rotors any longer at that point.
If I am performing a water based auto-rotation, there is quite a bit more to worry about. The first thing I have to worry about is that I won't be able to perform a running landing. This means that I'll have less margin for error in my auto-rotation. The second problem is that as soon as I set the helicopter down into the water, it's going to start sinking, and I'll still be strapped in. If I jump out, I'll have to worry about hitting the rotors (or the rotors hitting me) as the helicopter sinks. This means that as soon as I hit the water, I have to remain in the aircraft, and I'll pitch the cyclic over in the opposite direction from the side I'm sitting on, so that I can stop the rotors and still exit the aircraft on the surface side rather than underneath the sinking helicopter. This gets more complicated in the case of passengers, I would encourage them to jump out before the helicopter hits the water, but after my flare, so that they can clear the aircraft and I don't have to worry about them. I only mention it as an additional complication I'd potentially have to deal with.
All of this complexity is a large reason why most of the helicopter pilots I know really do not like flying very far over water. When I'm flying over the water, I tend to stick to an area near the coastline specifically for this reason.

When I'm flying over the water, I tend to stick to an area near the coastline specifically for this reason.
With only one engine, that's a good way to look at things.

The question refers to a "boat hull equipped" helicopter. Does that change any of your points about running landings and sinking? I mean... I figure the point of a "boat hull" is so that it doesn't sink, right?

Boy, the “having the pax jump” thing is a scary thought.  By the time you’re low enough for them to bail, you’re going to be tail-rotor low (yikes) and likely about to have a main rotor blade disintegration as you roll into the water just feet from where they are.  No doubt the risks to staying in the helicopter are large, especially smaller, higher-CG helicopters that roll so easily in the water. But I wonder if the risks of jumping don’t outweigh the risks of rolling.  If there’s time, is it better to have them “ajar” their doors, and look down and mentally rehearse how unbuckling will work?

Let's separate the terns you've used in your question and comments.
Emergency descent
This is a very ambiguous phrase and doesn't really describe a flight phase or operation whereas auto-rotation and water landing do.  The pilots operational manual will simply say something like "land as soon as possible".
An emergency descent means that you have a problem which requires you to get on the ground as fast as possible, not including all engine failure since that is an auto-rotation.
The fastest way of getting any helicopter down is to enter auto-rotation and if I really had to get down fast, that's what I would do.  In the last few feet though, I'm going to get the engine(s) driving the rotor again so I can make a normal landing.
Auto-rotation
A true auto-rotation does not have an option of a powered landing, since by definition you have no engine(s).  If you have power, then it's training, practice, testing or precautionary.  Everything you do until the last fifty feet or so is making sure that you manage the rate of descent and airspeed to keep the rotor in the operating range.  The end goal is to arrive at fifty foot with enough airspeed to be able to flare so that you can trade your kinetic energy from airspeed to kinetic energy in rotor rotation so that you can increase pitch and therefore lift and therefore drag in the flare, whilst feeding in kinetic energy to stop the drag from stalling the rotor.
It's all about energy conservation and conversion.  You start off with some potential energy (height and mass) and some kinetic energy (airspeed and rotor RPM).  An auto-rotation instantly starts converting potential energy into kinetic energy - you start going down.
Take two identical helicopters flying in identical conditions except one is at minimum weight and one at maximum weight.  The one at maximum weight has more potential energy since:

PE=MxGxH

Where M is mass, G is the acceleration due to gravity and H is height.
Both helicopters enter auto-rotation.  Quite soon, both will stop accelerating downwards and become stabilised with a constant rate of descent.  The heavier helicopter though needs more lift to support it's weight.  The airflow in auto-rotation always comes up through the disc from below. You can gain the extra lift by allowing the rotor RPM to increase or you can increase the angle of attack of the blades by raising the collective. The rotor RPM always have an upper fixed limit so in practice, the only way to gain the lift, and support the extra weight, is to raise the collective to prevent the rotor from over-speeding. You also have a limit to how much pitch you can use as the angle of attack must remain positive to the relative airflow from below.  If you try to reduce rate of descent by continuing to raise the collective, you will lose lift as the angle of attack moves towards the critical angle so eventually, you reach a steady rate of descent, greater than that of the lighter helicopter.
Since the heavier helicopter is now generating more lift, it is also now generating more drag which can only be overcome by converting more potential energy into kinetic energy. Therefore, the heavier helicopter will use up it's potential energy faster and, since the rotor cannot be 100% efficient, this means that the rate of descent will be higher than in the lighter helicopter.
(It's a very complex topic beyond either the scope of this answer or, to be honest, my complete understanding but some helicopters, in some weight ranges in some flight conditions can descend more slowly when heavier due to rotor efficiency and the size and location of the "auto-rotative driven region" vs the stalled region of the disc).
So, when you arrive at the bottom of the auto and it's time to flare, you have a high rate of descent in a heavy helicopter and the only way to arrest this is to increase collective pitch, to increase lift and increase drag.  The only power available to you is in your kinetic energy so you must trade airspeed for rotor RPM in the flare.  So a higher rate of descent means you need a higher airspeed to make a controlled power-off landing.
All of that explanation was only to explain that a heavy helicopter arrives at the bottom of an auto-rotation with a high rate of descent and high airspeed.
I couldn't find a video of a Sea King auto-rotating, probably because heavy helicopter pilots don't practice auto-rotations like private and sports pilots do.  But here is a heavy helicopter coming in to land.  Notice how fast it's going when the flare starts, how high it is and for how the long the flare is held.  Even after that, it still lands with a significant rate of descent and airspeed.
S-92 helicopter autorotation (power off landing)
In a light helicopter, especially with some wind on the nose, a good pilot can land with zero airspeed and just enough rate of descent to gently touch down.  You could not do that in a heavy without a lot of wind.
Water Landing
Now for the unclear bit.
If you mean landing on water at the end of an auto-rotation from normal flight, then clearly, a heavy is not going to fare well with that arrival so unless you have no choice, you're going to go for firm ground.
If you mean a hovering auto-rotation, assuming that you are within the limits of the height-velocity curve, then you have no choice and it's not going to end well unless you have flotation devices which are deployed and you make a really good touchdown.
If you mean the "water bird", then this is neither an emergency descent or an auto-rotation but is a single engine landing, therefore powered, where you use the boat hull shape of the fuselage to arrive at a precise speed and pitch angle to allow the hull to plane and cushion the landing to enable the helicopter to settle into a float.
What a long way of saying that emergency descents, auto-rotations and water bird landings are very different things and therefore not analogous.
Phew.

Sorry for the confusion. I made an edit to the title and added paragraphs trying to explain better. Basically what I want to know, is landing on the water (either powered or unpowered) a different procedure than on land? AFAIK you just hover down, but maybe the flares are supposed to go differently on the water for some reason?

I really don't think you need that much detail on autorotation, it's good but not relevant and too long.

@Robert I beg to differ, it's entirely relevant.  The size and weight of the helicopter has a very direct bearing on what kind of landing is carried out and where. I added the detail since, through experience, I know that if I don't the answer is less complete and will prompt further questions in the comments and, smart a** comments, such as "a heavier helicopter does not always have a higher ROD".  Many lay people and non-helicopter drivers also fail to understand the basics of auto-rotation.

A further point is that the Sea King is uniquely suited for a water landing due to its boat hull fuselage shape.  The Sea Hawk, Lynx, or Sea Sprite, for example, do not have the same hydrodynamic shape and thus will have significantly more trouble trying to do the same thing a Sea King can do.

Just a short addendum here:
the biggest part of an helicopter's weight comes from the engine+transmission+main rotor assembly. For safety and structural reasons, those parts are normally packed together and located in the upper part of the fuselage. That means that as soon as an helicopter lands in water that big mass wants to go down making the helicopter tip over and sink. This really nice (and scaring) video shows the training in the escape procedure used in these cases:
STRAPPED INTO A SINKING HELICOPTER (with U.S. Marines) - Smarter Every Day 201
Obviously this happens unless the helicopter has been specifically designed to land in water or is equipped with a floatation system, either fixed or inflatable. An example of the former is the Sikorsky Sea-King:

The following pictures show instead fixed and inflatable floatation system respectively as used on the R44:
fixed floatation system on an R44

inflatable floatation system on an R44

(All images copyright wikimedia commons)

That’s a very instructive video, thaks for linking.

Incomplete answer as your question is mostly answered by the other posters.
Emergency landing in a light helicopter equipped for water landings - normal ish landing, if it is glassy with no good ground reference, a slow forward speed with continues descent until you touch the water
Emergency landing in a light helicopter NOT equipped for water landings don't fly away from shore simple. - same as above, coming to a stable hover with skids in water, setting it slowly down [with doors open etc.] when it is sort of floating, hard right or left cyclic and roll it, when rotors stop [they quickly will] unbuckle seat belt and get out.
Auto - water with good reference, ie, beside the shore, normal flare and landing, when down, roll sideways, and get out.
Auto - Water with bad reference, ie, whitecaps, floating log etc, flare higher than normal and plan to hit harder, when down, roll sideways, and get out.
Auto - Water with no reference, ie glassy water far from shore, set up for no flare auto 30kts [check your POH] when you KNOW your low enough pull collective, do not flare, better to hit really hard than to pull at 150 feet. if your still alive, get out. Some POH's may have detailed instructions for this scenario
No experience with helicopters planning on landing on water, or muti-engine.


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How do mixed-bag-of-hardware military coalitions handle communications, IFF, et. al. interoperability?
Back in the Cold War era, there were Western-allied air forces on one side using NATO-standard communications and IFF equipment, and Warsaw Pact-allied air forces on the other side using Soviet-standard communications and IFF equipment, and they never met except as adversaries in battle.  Both sides exported equipment aggressively, but only to countries they considered friendly enough.
Now, the Cold War is basically over.  The major players (US, Europe, Russia) continue to make sales in the export market for their equipment, while more independent-minded folk in minor powers are not restricting themselves to a single source, leading to cases like the Royal Malaysian Air Force where Flankers and Hornets share the same flightline.  However, in that case, the RMAF has the power to order or retrofit everything with the same comms radios and IFF gear, and probably has done just that so they can all talk to each other and figure out who's who securely.
However, the fall of the Cold War has also meant that fighting has centered around coalitions to neutralize specific belligerent "hotspots" (the First Gulf War being a prime example of this).  This means that in some areas of the world, it is quite plausible that you have an air force with MiGs and an air force with F-16s on the same side of the fight, trying to work together against a common foe.
This raises a question though: how do you get everyone on the same side of the fight on the same page regarding communications and IFF gear?  Did the Russians reverse-engineer and adopt the NATO COMSEC and IFF standards, relying on key control to keep the streams from mixing?  Or would there be much furious last-second swapping of radios and IFF boxes in such a coalition situation to allow the MiG pilots and their F-16 pilot friends to talk to each other securely and to know where each other is?  Would the communication situation fall on ground relays to straighten out?

The RMAF and IFF; reminds me of this bar story.
On the first flight test for a new IFF radar of the RMAF, some 20 odd years ago, the two flight test aircraft collided after turning each other's direction immediately after take-off and crashed. A month later, after the dust settled, they tried again. Learning from their previous mistake they planned on taking-off shortly after each other instead of next to each other. This time however, the IFF transponder in the first aircraft wasn't configured correctly which caused an alert in the cockpit as soon as the aircraft was seen by the radar. ...

...The pilot, not familiar with the alarm, thought of an engine failure and aborted the take-off. His wingman, following closely behind him, noted this too late and crashed into him.
The radar engineer in charge of installing IFF on the radar, a friend of mine, wasn't impressed with the RMAF's performance and remarked: "Soon you won’t need IFF anymore because you won’t have any friendly aircraft left".

I read, but cannot retrieve articles, that the data links between allied have been a failure during the first Gulf conflict, AWACS command posts being unable to play the role of coordination/integration between allied forces they were supposed to play. I'm not an expert on military matters, so feel free to correct me.

@mins There is a difference between "difficulty" and "failure."  I will suggest that you have been the victim of a bit of hyperbole.  The Data Link Interoperability problem within NATO has a long and distinguished, and colorful history in part due to the NATO STANAG being built for lowest common denominator/interoperability, and various nations like the US forging ahead with better Link than the LCD.  This is in part due to various nations not spending their 2%, but also due to how expensive some kit is versus the GDP and tax base of some NATO nations.

Never, ever, let someone maneuver to your 6 unless he is your wingman.

You do multinational exercises
This is a good question that was in part answered for some NATO and neighboring countries under a program called the "Partnership for Peace."  That's one of many methods various nations have used to exercise and practice interoperability, which includes figuring out the communications play.  SEATO was another regional interoperability vehicle, as is OAS.
The first exercises I got involved with (mid-to-late 90's) were Non Combatant Evacuation Operations and Peace Keeping exercises with former Warsaw Pact countries.  It was very much a crawl, walk, jog, trot, run, sprint development model.  It doesn't happen overnight, but exercises, practice, training, and extensive military-to-military contacts go a long way toward identifying what is and what isn't interoperable.
As to "swapping IFF boxes" that's rather pointless.  You can develop procedural control for (Return to Force)RTF procedures, and you  have to accept that in a lot of cases you can't use encrypted comms.  What you do as a group is develop a comm plan that uses generically common radio equipment (do we all have VHF? Yes? No?).  Each operation will have its own wrinkles and details.
You build on previous multilateral operations like UN Peacekeeping
Since the UN stood up in 1945, over a hundred nations have some experience in contributing forces and equipment to UN Peace Keeping operations, and to other UN combined operations (like the UN Operations in Bosnia before 1995, or the peace keeping in the Sinai forever).  With this experience, nations have built up some "lessons learned" as a base for working with each other.
There is no easy button; you have to do formal planning
You don't just show up with your five helicopters, say "We are here!" and get to work right away.  Your air force staff will be in contact with the other countries' air force staffs and a communications plan will be established.  A general layout for forces will be agreed, such as "who goes where?" and it takes detailed effort to establish that.  A whole host of other details get ironed out like how to get the fuel to the airfield. (See problems in Rwanda, 1994, for a prime example of the nightmare that can be).
Military Planning, both long term and short term, is an art in and of itself. (It's painful -- been-there-done-that-got-the-t-shirt -- but it must be done).

An Operational Example
Here is an operational example of a situation where there was no lead time, no preparation time to plan for a response to a politically tense situation, and yet there was need for close coordination between those directing the fight and those fighting the fight.
I was an A7-E light attack pilot on the USS Nimitz. Sadat had been assassinated in 1981 and the boat was directed to a station off the coast of Lebanon. Once getting there the A7's were kept on 5 minute alerts throughout the night. Fully armed with APAM, I can remember sitting in the aircraft at 3:45 in the morning. It was difficult to keep my mind and body ready for a five minute launch into a night attack in an unfamiliar territory and minimal guidance.
But we had our Forward Air Controllers (FAC) and they were good. In fact, all of the controllers I had throughout my 2 tours were very professional, and excellent at what they did. The FAC is in the country, on the ground, maybe in the air, directing the aerial attack assets. A brief description would be that they keep the aircraft in holding patterns, bring them in on specific targets, and further direct the attack by helping the pilot identify the target. Think of your own in-flight/approach controller managing a combat attack.
They had come out to the ship earlier and briefed the pilots on radio communications and procedures. I remember being given a bunch of colored knee board cards in this really small, cramped courier type. Made me smile, because at night in the red light of the cockpit, coming in for a strike they would have been unreadable. So I studied them when I wasn't flying and memorized their information. They were loaded with stuff about procedures, holding points, timing, call signs, authentication, etc.
Having the face-to-face communication was a great way to prepare for the engagement where several countries, and different agencies, were coordinating a military response.
Practice & Discipline versus Deceit
Here is another way to frame the question. What often gets neglected in these sorts of discussions is the role cunning and deceit play in combat. I remember coming through my first air-combat-maneuvering training syllabus. Outside the ready room were quotes from famous Aces. I remember a quote from the top Japanese Ace in WWII. He said that for 90% of his kills his adversary never saw him. He just flew up under their 6 and shot them down.
The practice and preparation I experienced in the Fleet seldom included exercises which exposed weaknesses to "infiltration." In the following post there is an interesting example where coordination within a battle group was compromised with deceit and patience. In this case there were multiple assets being directed by unfamiliar controllers: What is the average speed used by modern jet fighters when in dogfight?
I heard this sort of thing talked about only two times. Once was with a new pilot who came into our squadron. He asked, "Why are we using the same tactics for a war-at-sea strike that we have used for the last 20 years, and with which the Russians are now quite familiar?" His question was never addressed by our command, although he and I discussed the problem in depth and attempted to address the issue ourselves.
Situational awareness and the ability to think outside the box is what keeps a pilot alive. A dogfight I participated in emphasizes this fact. Effective communication only gets you so far. Is it possible to perform a spin recovery in IMC?
More importantly, the second time I heard this topic brought up was when we were preparing for a particular real world engagement. It was noted that the Russians would have English speaking agents on the Net. They wouldn't be speaking perfect English, but native English, and would be difficult to distinguish from other American pilots. That is the threat.
You can have the most secure communications, and I will find a way to deceive you, and then the complacency your security has encouraged will be your worst enemy. In this case discipline and preparedness protect the flight. Practice like you will fight.

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Why aren't modern helicopters quieter?
Living closer to a Sikorsky factory, I can attest that modern civilian (and military) helicopters are quite noisy. In any case, they are considerably louder than light aircraft, turboprops, and even some jetliners.
This has resulted in helicopters being banned from approach and landing in some jurisdictions.
It seems though, that the techniques for sound mitigation of helicopters have been known since at least the 1970s (The Quiet One) let alone the stealth helicopters used in the Bin Laden raid.
That is,  part of the solution appears to be to use more rotor blades while decreasing the speed of the rotors. There are also modifications to the tips of the rotor blades that while they may be classified, could be deduced and reverse engineered.
Why are helicopters so loud and why aren't contemporary helicopters quieter given what we know and how far we've come in materials sciences and aviation design?
Or is a question of economics (quieter == less economic) or politics (national security prevents the use of sound-mitigating technology) or something else?

I may be wrong, but I thought that the forward moving tip breaking the sound barrier was one of the limiting factors in both a helicopter's forward speed as well as the rotational speed of the disk. i.e. having a tip break the sound barrier was a Bad Thing™ in a helicopter.

@Freeman - That's why lowering the speed of the rotors is (generally imagined to be) one of the stealth technologies used in the OH-6 as well as Bin Laden copters. But why this isn't commonly used is part of my question. Seems a no-brainer, especially when the folks living near Bin Laden's compound said they didn't hear the copters until they were overhead. If only that were true for general aviation! :-)

I was thinking that supersonic blade tips creating shock waves that would hit the fuselage and other blades was a more serious issue than just creating noise (i.e. vibration, loss of lift, etc). Maybe I need to ask another question...

One aspect:  More blades => smaller disc radius => higher disc loading => less efficiency => reduced range => higher fuel consumption => higher weight => reduced autorotation performance.  I'm guessing that all of the above are a design compromise trade-off against market demand and the military don't care much except for the stealth bit.

I think there is a psychological component at work as well.  We might ask why many motorcycles, ORVs, jet skis, cars & trucks, and even (or perhaps especially!) yard equipment are far noisier than they need to be, and why some owners spend money on aftermarket mods to make them even noisier.  It's because to a certain type of mind, noise equals power!

For what it's worth, MD Helicopters claims their elimination of the traditional tail rotor results in a significant noise reduction. See http://www.mdhelicopters.com/v2/notar.php.

@FreeMan - thanks for your question. I did some googling and it does look like my assumption about supersonic tips may be wrong. I've asked a separate question about that.

I have to be honest, when I first read the question, this was the first "stealth helicopter" that came to mind... :)

The physics of helicopter flight are very complicated.  It is that way so more people can make a living.  The truth is, helicopters don't actually fly, they just beat the Earth into submission.

Need a follow up there: Chuck Norris can run on the blades of a helicopter to make it fly higher.

The helicopter was invented after Chuck Norris was observed doing 8 roundhouse kicks a second.

It's an R&D and therefore ultimately a cost-benefits decision. Quieting techniques for helicopters aren't as simple as "hush kits" for fixed-wings, because the engines and rotors are a much more integral part of the entire airframe. So, to incorporate these features into a civilian helicopter is a much more involved process than swapping blades on a turboprop or putting hush rings on the exhausts of older airliner engines. Changing the physical properties of the rotors (lift, induced drag, moment of inertia) requires compensating changes in the engine; shorter blades reduces lift requiring higher RPM, more blades increases MOI requiring more torque, etc. Those engine changes will in turn require changes to tail rotor characteristics, and may also require frame and body tweaks, etc etc.
Most helicopters in civilian use today, like the Bell 200-series, Sikorsky S-70 family, Dauphin etc are Vietnam-era designs, almost unchanged save for a few minor tweaks along the way like incremental engine improvements or new avionics systems. They're still being built and flown because they work; take the rigorous proving tests required of any aviation system and the maintenance and inspection requirements of fixed wing aircraft, and add an additional level of caution for the inherent instability and lack of fail-safes in the design of a rotary-wing aircraft, and you'll have some idea of what it takes to get a new helicopter vetted for GA use. If the helicopter's engine cuts out or throws a prop blade, you can't just glide down to the ground the same way as a fixed wing. So, any changes you make to a basic, proven design would be subject to extreme skepticism and testing by both aviation regulators and owners, to say nothing of starting from scratch. This decreases initial sales of a new model in the civilian market, increasing the payback period of your R&D to develop this new design.
Second, in part due to this hesitance by GA to adopt new chopper designs, the funding availability to research and develop any new civilian helicopter design is dwarfed by military interests. As mentioned, practically every design you see in the skies today has a military pedigree and was designed primarily for that purpose, then civilianized for GA use (a notable exception being the Eurocopter Dauphin 2 which was established in the private and law enforcement sectors before militaries took a look). As a result, a lot of the newest stealth technologies available for helicopters, including noise reduction, are heavily classified. For instance, the modifications to the UH-60s used in the Bin Laden raid; civilians really only know that stealth Blackhawks exist, as what they look like and their performance/noise characteristics are all still top secret. That hasn't stopped the human imagination; artist impressions including the concept art for Zero Dark Thirty abound.
In addition, the civilian quiet-copter technologies that aren't classified, like Eurocopter's "Blue Edge" blade design, are mostly protected by international patent. Bell or Sikorsky can't just reverse-engineer the Blue Edge blade and stick it on their choppers. An independently-developed design must be different enough to be patentable, which probably means it will be more complex, and complexity is a bad word in aviation because it means increased probability of system failure or operator error. The other avenues would be to either license Blue Edge from Eurocopter (increasing the cost of a Bell or Sikorsky Blue Edge-equipped model) or wait 15-20 years until the patent expires (further delaying widespread adoption of the technology beyond one brand).

"Bell or Sikorsky can't just reverse-engineer the Blue Edge blade" They don't need to: to patent something, you have to fully disclose the design.

Your be surprised how much you can obfuscate how a patented mechanism really works, including various dynamic variables like how the design scales for optimal performance at different blade lengths, which would be necessary to put the blade on different helicopters.

Helicopters glide a bit worse than fixed wing aircraft (and call it autorotation), but compensate by still being able to land with no or very low forward speed at the end, so engine failure is not a fatal problem. They do have a single point of fatal failure though, the rotor hub.

Not everything is classified: Scissor-blade tail rotors is one technology to reduce noise: https://aviation.stackexchange.com/questions/43279/what-are-the-benefits-of-a-non-orthogonal-scissor-tail-rotor

Wikipedia mentions even more technologies: "In order to reduce their acoustic signature, the helicopters (N351X and N352X) received a four-blade 'scissors' style tail rotor (later incorporated into the Hughes-designed AH-64 Apache), a fifth rotor blade and reshaped rotor tips, a modified exhaust system and various performance-boosts." (https://en.wikipedia.org/wiki/Hughes_OH-6_Cayuse#Vietnam_War) All these technologies are known for a long time, but not used in civilian helicopters.

And https://en.wikipedia.org/wiki/NOTAR and https://en.wikipedia.org/wiki/Fenestron are two commercial technologies used to reduce noise from helicopters.

A large part of the noise produced by rotor wing aircraft comes from the drive system. The engines themselves make quite a bit of noise and it is technically infeasible to make them quieter.

Presumably the same hush kits on turbofans could be used -- or other muffling technologies.

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Why don't planes have a reverse propeller as air brakes?
When I first heard the word air brakes it came to me that maybe the engine start rotating in the opposite direction for that, but since that's not possible, why can't we have an extra engine for air braking?
Did I just invent a new braking paradigm?

Here's an extreme example of an RC plane using variable pitch props to reverse thrust: http://www.youtube.com/watch?v=o_o_HT2ZaKQ the setup of the prop is something like this: http://www.hobbyking.com/hobbyking/store/__6298__variable_pitch_prop_motor_set_9_type_b.html. As mentioned in the answers below, real planes can do this too but obviously not as extreme.

I would consider Google first before thinking you have just invented a paradigm shift. A simple search would have revealed that this has been around for many, many years.

actually I get the idea from despicableMe, when Dr. Nefario goes to rescue the girls and he shoots his clamp on Vector's ship, then reverses his jet engine

How do you cover the cases when you are short of fuel before landing, or the engine has failed? No engine = no brake capability.

@slebetman Interesting find on the Hobbyking prop, but the one in the video is a fixed pitch prop on a reversing electric motor.

@Pilothead: No. It's a variable pitched prop. Not reversing the electric motor. The description from the video itself says "A Dennis design-- lazer-Made it out of EPP, Joe milled it -and VPP'ed It". VPP stands for variable pitched prop

@Pilothead: Pause the video at 20 seconds and you can see the servo going to the motor assembly controlling the prop pitch

@slebetman Thanks

Planes with a variable pitch propeller can have a pitch range which twists the blades such that they provide reverse thrust. This started to become a common feature on high-performance airplanes in the 1930s.
Not all variable-pitch propellers can produce reverse thrust, however. The constant speed props on small propeller aircraft usually don't, because the energy of the plane after touchdown would be too small to justify the added complexity. On big airplanes which need short landing distances (the C-130, for example), they are a standard feature.
In 1936, Heinkel produced a sleek dive-bomber, complete with retractable landing gear and a reversible pitch propeller which could be used as a dive brake. The He-118 was test-flown by Ernst Udet, who did not pay attention at the briefing and went on to overspeed the airplane in a dive, destroying the prototype in the process. Thus, the clumsy, fixed-gear Ju-87 was chosen to be the "Stuka" and the He-118 would end as an inspiration for Japanese aircraft designers.
The Pilatus PC-6 Turbo Porter is a favorite with skydivers because it can put the propeller into reverse even in flight. This allows for spectacular descents and enables it to be safely back on the ground before the skydivers it dropped a moment before have landed.
As usual, Wikipedia covers the topic well, so here ist the link.

For completeness, reverse thrust on propeller aircraft is called "beta range", if anyone wants to google it.

TBM 850 (and family) all come with beta range standard. It's a turboprop that can cruise at 320kts TAS, but has to be able to land in single engine GA performance numbers. So they fitted it with massive flaps and spoilerons to reduce the stall speed and reverse BETA (ß) range to help it stop quickly.

Peter Kämpf's answer covers the systems that prop planes actually use for reverse thrust.  The alternative presented in the question is a total non-starter for several reasons.

It's a complex and very heavy system.
Unless there was somewhere on the centreline you could mount this engine and prop, you'd need not just one of these things but two, for balance.
It causes drag while it isn't being used, which is all the time except immediately after landing.
For certification, aircraft have to be able to stop using just the wheel brakes, anyway, since the complexity of reverse thrust systems makes them likely to fail.

Exactly, the added benefit would be negligible (especially since many variable-pitch props and almost all jet engines are already capable of producing reverse thrust) while the costs (in design, manufacturing, maintenance, and fuel costs as well as in performance) would be enormous.

Jet aircraft (especially large transports) are often equipped with thrust reversers. Not a requirement for landing, but takes some of the load and wear off the wheel brakes and tires. They have them because of the high landing speeds and large masses they have to bring to a stop in a limited amount of space.
Propeller aircraft don't fly as fast, so landing speeds are not as high; for the most part, braking requirements are easily met with wheel brakes.
Even if reverse thrust was beneficial, reversing the engine's rotation would be totally infeasible; engines are simply not designed to run either way round, and any system which would decouple the engine from the propeller (as would be needed if you wanted to implement a "reverse gear") would be a safety/reliability nightmare, not to mention heavy and expensive.
Engines are a significant part of the total weight and cost of an airplane. Carrying around an extra just for reverse thrust would be impractical for cost/weight reasons.
Many propeller-driven aircraft are equipped with variable-pitch propellers, including many light aircraft; in theory, any of them could have a "reverse" setting, but (as described in one of the other answers) you would only see it on larger aircraft designed for operation on short runways.

"Combat zone landing" procedures for military transport aircraft, which are often turboprops, calls for full reverse after landing to stop in as short a distance as possible (less than 3000ft for a loaded Hercules, for example).

The use of reversers on an aircraft with Carbon brakes can have an adverse effect on brake wear. Since autobrake systems target a deceleration rate, the use of reverse can cause the system to cycle the brakes on and off, increasing wear.

There are some engines that can run in both directions.

@Johnny Yes, but those need to be stopped and then restarted in order to do so.  You can't do that reliably, within a couple of seconds, as would be needed during landing.  There were a few prototype steam-powered planes which did that, though, because reversing a steam engine is very easy.

Most jet airliners and some propeller-driven aircraft (chiefly those designed for short-field operations) do in fact have some sort of thrust reversal system already.  There is no need for an extra engine for this purpose, which if fitted would be a weight problem and a maintenance nightmare.
The thrust reversers on jet airliners are pairs of ducts which, when deployed, intercept the normal jet exhaust and redirect it forwards.  The reverse thrust produced is somewhat less than the forward thrust available, but it's plenty to permit a powerful deceleration, and it's particularly valuable on wet and icy runways where wheel brakes aren't very effective.
The Harrier "jump jet" fighter is nearly unique in its ability to take off and land vertically, and is capable of using the same thrust-vectoring control in combat.  By suddenly turning the thrust backwards, the aircraft can decelerate very sharply in midair, potentially moving it from a defensive position in front of an enemy to an offensive position behind it.
The Piaggio Avanti is a particularly interesting civilian turboprop, with an unusual "three surface" wing layout, pusher propellers, and very clean aerodynamics.  It can also land and come to a halt within 500m of runway, thanks in part to reversible-pitch propellers.  This short-field capability contributes to its lower operating costs versus corporate jets of similar size and performance.


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Is there a best number of wheels per bogie?
I understand the need of having more wheels when the aircraft is designed to be heavier. The number of wheels per bogie and the number of bogies differ from an aircraft to another. For example, there are some main landing gear compositions (from the lightest to the heaviest aircraft):

A320/B737: two 2-wheel bogies
A330/A340/B767: two 4-wheel bogies
DC10/MD10: two 4-wheel & one 2-wheel bogies
B777: two 6-wheel bogies
B747: four 4-wheel bogies
A380: two 4-wheel & two 6-wheel bogies

Given those examples, I guess the maximum number of wheels per bogie is 6 and thus given the number of wheel of the B747's main landing gear, it could have been composed of fewer bogies with more wheels on each (e.g. three bogies: two with 6 wheels and one with 4 wheels). So I wonder if there is an optimal number of wheels per bogie, and if for a given number of wheels it is better to add bogies with fewer wheels per bogie.

The answer depends on who you ask. The structural engineer would prefer a single wheel, and as few legs as possible, while the aerodynamicist would vote for 8 or 12 wheels, because then the bogie has less frontal area and can be accommodated even in thin wings.
Generally, less than two wheels would compromise redundancy, and more are needed to reduce local ground pressure. Remember that the A-300B was initially not allowed on some US airports because of a slightly higher ground pressure. See this answer for more.
Modern designs use two wheels per strut for redundancy when the aircraft is light enough and replace them with a bogie with as many wheels as necessary to carry higher aircraft masses. The A-340-200 has gained a central strut with two wheels (A-340-200/300) rsp. a 4-wheel bogie (A-340-500/600), while the equally large, but lighter A-330 uses just the two 4-wheel main gears without the central gear. Airbus could have used two six-wheel bogies, but preferred to keep commonality with the A-330 gear.
More than 6 wheels per bogie is very unusual, but there are exceptions: Since the Tu-144 had no thrust reversers, all braking had to be done by the wheel brakes. Additionally, the bogies needed to fit between the intake ducts inside the nacelles, so the final design used 8 wheels with the large housings of the disc brakes sticking out towards the center of the bogie. As @Zeus points out in the comments, these were more precisely 4 dual-tire wheels per bogie: The two tires on each side were mechanically linked.

Tupolev Tu-144 landing gear (picture source). This particular aircraft is parked in the back yard of the Aviation Institute in Kazan. If you want to know how this 8-wheeled monster would fit inside a supersonic airliner, read the answers to this question.

XB-36 (first prototype of the B-36) taking off (picture source)
Convair experimented with different tire sizes for the B-36 and even a tracked version. The initial single wheel design resulted in the biggest aircraft tire ever produced, but in the end the conventional (but novel at this time) 4-wheel bogie was preferred because the single wheel had too much ground pressure for most US airbases. Earlier airplanes had used 2-wheel bogies (Tupolev TB-3, Junkers G-38) or a pair of wheels left and right of the strut (Douglas DC-4). Very early large aircraft had to use what was available, so the Caproni 42 bomber of 1918 below had as many main wheels as the TU-144 half a century later (picture source). The Zeppelin Staaken giant aircraft of WW I used the same number of main wheels on a single axle for soft field operations.

When Air India ordered their first A320s, they were afraid the main wheel pressure would be too much for some Indian airfields. Airbus produced a version with a four-wheel bogie instead of the usual two-wheel main gear of the regular A320. When they later ordered the A321, India seemed to have improved their airfields, because the newer planes use the regular two-wheel gear which is mechanically simpler and needs less maintenance.

Air India A320 (picture source)

As far as I remember, technically Tu-144 had 4 dual tyre wheels on each bogie.

@Zeus: That is correct; the two tires were linked and could not spin independently of each other. Thank you for pointing this out!

The problem is not one of the bogie itself; a bogie could also take 10 wheels or more (An-225). But the superstructure of the aircraft has to withstand the forces expected during landing; and of course the landing gear should be retractable into the body for fuel efficiency and speed. Depending on these constraints (and possibly more, like re-usability over a plane family), aircraft designers have to select appropriate bogie configuration accordingly. Depending on the priorization of the constraints and depending on how the plane evolved from is predecessors, different aircraft designers could come up with different conclusions/solutions even for the same airplane, let alone different airplanes with different superstructures.

The An-225 appears to have 7 bogies on each side with 2 wheels each.

@fooot: I would even say it hasn't any bogies, but pairs of wheels on a trailing axle.

There is no "best" overall, eternal, number. Any question asking for a mythical "best" anything can't ever come up with a specific answer as none exists.
The best undercariage configuration is that configuration which most effectively takes the weight of the aircraft and distributes it to the ground without overloading the surfaces it's moving over.
And THAT will depend highly on not just the size, shape, and weight of the aircraft, but the operational characteristics, landing and takeoff speeds, etc. etc. etc.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Why are the wingtips of sailplanes painted red?
Modern (plastic) gliders are painted white (I suppose for thermal reasons), except for some shapes on the fuselage (generally blue or red, but it seems the color does not matter). And the aircraft registration is dark, I suppose for maximizing the contrast.
Now, for the wingtips I noticed that there are two possibilities:

white (nothing special)
red marks

My question is: Why red? Is there something special that is preventing the wingtips from being painted in another color?

If one was red and the other was green I'd say it is to match the navigation lights.

I instantly googled other colors https://en.wikipedia.org/wiki/Wingtip_device#/media/File:Grob_G103C_Glider.jpg

Additionally, many sailplanes seem to be a light yellow color, some are orange, and here's one https://s-media-cache-ak0.pinimg.com/736x/47/d6/74/47d6749555aeab495ab5ee054679c648.jpg

Other colours than red are used too. Our club has two ASK-21s; one with red markings and the other with blue markings.

@fattie OK, I did not search hatd enough. Nevertheless, red is far more common.

They are not painted red. They are just very sharp, and ground personnel sometimes isn't.

Personally speaking I like red wingtips because it makes seeing the wingtips a lot easier.
During a winch launch it is critical to keep the wings level which is easy in mid flight as you have a lot of time to look about and check the instruments, on the ground run of a winch launch (where fatalities do occur) the instruments are useless due to the bumping and you have no time to check anyway. Having the red wingtips means that you can see them in peripheral vision with a minor turn of the head.
Once off the ground in the launch, again it is important to keep level, there is no horizon (actually there is but it is underneath the aircraft and useless) and the horizon indicator may still be wobbling and takes time to check. Peripheral vision and red wing tips win through again.
Most of my club gliders have red wingtips but some have fluorescent colours.

I strongly believe it's for security reasons to see the glider from longer distance. Anything that maximizes the contrast will do. We have a twin with blue strips, a Ka-8 painted with green strips (ok, that's not plastic..)

Maybe this is an ignorant layman question, but how does seeing the wingtips help keeping the plane level when there is no horizon as reference? The two times I've experienced a winch launch I had the feeling that my vestibular system wasn't much use. :)

@Emil. Humans can tell what is up instinctively, we have a built-in gravity sensor that is fairly accurate. We can all sit up straight. Looking out at right angles is also fairly simple for most people. In launch I can look out at right angles to gravity and if one wingtip is at a different height to the other then I can correct it. Once the cable is off and you are free to bank and turn then the real horizon is better as the centripetal force confuses the body. On the ground run you can compare the wing tips against the ground.

As a side note: the color authorized by mfg for painting is on the manual. Usually is somehow contrasting color for registration and a few super-visible colors for wingtips etc. Fuselage is only to be painted in white except for some (rare) exceptions where orange or yellow is homologated.

White is indeed chosen for thermal reasons. Gliders use epoxy resins which cure at room temperature and are then tempered at 60°C. This tempering shifts the glass transition temperature to something close to the tempering temperature, so the structure must not be heated under stress to more than this temperature. To limit heating by solar radiation, black surfaces should be avoided altogether and mostly red, blue or grey is used for markings.
The red wingtips were popular in the 1970s and were meant to improve visual identification when flying in hazy weather and close to clouds. They can be mostly found on gliders of this era. Since structural loads are lowest at the extremities, wing- and fuselage tips can be painted in high-visibility colors.

Schempp-Hirth Cirrus with red wingtips but no markings (picture source). This makes it obvious that this is a British glider.

kind of off topic, but curious -- the lack of markings indicated British origin?

Everyone knows that red ones are faster! If you disagree, I'm sending the waaagh against you.

@cat: By Gentleman's agreement, the British authorities do not require markings, and the glider pilots behave responsibly, so there is no need to track miscreants down. And it works!

@BoundaryImposition I think the definition of "Gentleman's agreement" as an informal and legally nonbinding agreement is understood without virtue signalling...

In truth: to see the plane well, you need a bright color, not green (too similar to herb), nor blue (too similar to sky) . What is left?

@cat See the Air Navigation Order articles 24(2), 24(3), and 32(1). Of course other countries may have similar regulations.

@Pierre.  I agree.  Everyone knows red adds, like, 10hp.

Additionally reflective red or orange markings are required in Sweden when flying in winter conditions in the mountain areas. Stockholm segelflyg tradionally goes up North to Ottsjö, a bit outside Åre, for flying. Starting and landing on the lake ice. They (sadly I never made it there) fly in the upwinds created above the mountains when the wind comes from the right direction. Oxygen is used and I believe a few has reached 10.000 meters. It is supposed to be spectacular flying in wind rotors. Cold as well.


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Why does the zeppelin NT tail use 3 fins?
I don't if I use the correct vocabulary. By "fin", I mean the control surface at the tail of the airship. As a control surface, it contains a moving part.
As airship used to have at least 4 fins at the tail: an elevator (1 fin on the right side, another one the left side) and a rudder (one fin on the bottom and one on the top, I highly suspect both are useful to be sure at least one is in undisturbed airflow while cruising at any positive or negative angle of attack). This seems reasonable to ensure both redundancy and simplicity (not mixing pitch and yaw controls). Yet, aircraft with V-tail prove that we can reduce the number of mobile control surfaces located on the tail, but it does not seems to be reasonnable in all cases.
For the Zeppelin NT, only 3 fins are used. I clearly imagine the vertical one is used for yaw control only. The 2 others are shaped as an inverted V, thus I imagine they mix both yaw and pitch control as V-tail does.
What is the rational behind the choice of number of control surfaces? Why do the Zeppelin NT uses 3 of them?
Wikipedia provides few reasons I fail to understand:

it states less fins allow weight gains. for me, less fins implies bigger fins to provide the same authority, thus mitigating this point
it states the loss of one fin can be compensated by the 2 other. For me, the loss of one fin would provide a rolling moment when trying to compensate, and there is no other surfaces to mitigate it (on an aircraft, coupling is possible because it has wings)

2 is not enough while 4 is too many.

@user3528438 if 4 was too many, other airship would not have 4 fins.

For each airship, there exists an integer n such that n-1 fins are not enough while n+1 fins are too many. For certain airships n=3, such as Zeppelin NT.

@user3528438 then the question can be understood as "why n=3 for this airship?"

see my first comment.

@user3528438 your are saying 3 is the optimal number but not why 3. When I point out you don't give reason, you just basically say "I alreaddy told you because 3 is optimal, not more or less." without saying why 3.

How is that not a reason? Why do cars have 4 wheels? It'll be a valid question if you ask why 2 is not enough and 4 isn't necessary though.

@user3528438 this is exactly the question. It basically asks why not 4. I suspect you didn't read it.

The likely reason would be ground clearance.
While it is hard to guess the reason for a relatively minor design decision like this, it seems that using four fins would complicate the tail design. Specifically, the airship would need to have a lower gondola or an X configuration for the tail, which may or may not impose a weight penalty on the tail structure.
As you note V control surfaces tend to be a net weight penalty once you factor in the higher loads placed on fewer surfaces. However, when the choice is between four small surfaces and three larger ones, the latter is likely to be more efficient.
You also note that the loss of one lateral surface would create a rolling moment, which is true, but this is the one situation where an actual "pendulum effect" (sigh) can be observed: the buoyant lift of the airship is directed against gravity, so it has a stable equilibrium point with the gondola pointed down, which simplifies the control problem.
Without knowing the critical load cases for that tail, it is hard to give a definitive answer, but this stands out as a plausible one.

Actually, the Zeppelin NT has 4 "fins", if you include the gondola.
First, directional stability.  As with rockets, 3 fins are fine, and the symmetrical body or fuselage or gas bag or whatever airship captains call it also assists in this function as well.
You only need as much fin as is necessary to avoid uncomfortable stability overshoot.  This dog should not wag its tail.
Now for side force, which is dependent not only on fins, but the balance of area above and below the vertical center of gravity.  Because of the gondola side area, the 3 fin design actually may be superior in this respect.
One may want a great airship to point slightly into the wind.  Again looking at area fore vs aft of the CG, fins are helping here.
Any worries about 3 fins needing heavier structure for sufficient strength?  From the archives of aviation at airship speeds we have .... bracing wires!
The NT also has swiveling engine pods for control, and uses helium, making these "new technology" zeppelins safer than their more famous forebears.
Aircraft have successfully used 3 fins in the rear as well.

even counting the gondola as a fin, it has less fins than a common blimp

Counting the gondola as a fin is a mighty stretch: most of the mass is concentrated in it, making its effective equivalent fin volume almost nill.

@AEhere supports Monica it would be great to know exactly where the CG is, but I would agree it is near the bottom.  It would function as a "fin" for sideforce analysis.  These ships may have a mass/side drag profile similar to a hang glider.  They did anhedral the others a little.


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Can a registration number be reused indefinitely?
This answer highlights the fact that some registration numbers can be reused (here F-WZFD as a temporary registration number for newly built aircraft and 9V-SMF for a commercial airliner). This made me wonder how many times registration numbers can be reused and the existing constraints to reuse a registration number.

Is there a limit for the number of time a registration number can be reused?
Is there a minimum time between the decommissioning of an aircraft and the reuse of its registration number?
Are there registration number that cannot be reused?

I'm interested by all ICAO's member but if needed, the answer can be narrowed to the jurisdictions covered in the linked question, namely Singapore and Europe.

In what country?

@RonBeyer edited accordingly

Related: https://aviation.stackexchange.com/questions/3533/can-you-re-register-a-de-registered-aircraft-with-the-same-n-number


text:
How do pilot and copilot operate and coordinate the radio?
How do the pilot and copilot of a large commercial jet operate the radio when talking to ATC? Is there a push-to-talk button, or some voice-triggered mechanism?
Also, how do they coordinate it? How do they decide which pilot should be talking to ATC?

There is less radio work than you might expect.  It's only really significant in the busy phases of the flight when the will be a clear agreement, acknowledged by both crew, who is PF and who is PNF and who will work the radio traffic.

Both yokes have PTT buttons. Generally one person is in charge of the communications. In military aircraft that is often the task of the navigator in twin crew airplanes. I would think it generally falls on the pilot not flying the aircraft although both should be listening to the instructions.
The switch is on the front of the yoke (reverse side from what the pilot sees) and is generally triggered with the left index finger. Here is a pic from a yoke for reference. Note the button on the left upper part labeled "mic".

source
I have never seen it (but then again there are many radios I have not seen) but some radios may have a VOX (voice-operated transmit) option which would bypass the PTT. This however would be very irritating on the channels since there are lots of planes on a single frequency talking to the tower.

Okay but how do they designate who that one person is?

They talk before the flight. They decide who will be pilot flying and who will be pilot monitoring, which often does radio operation as well. If they wanted to toss coins to determine who does what, they could! :)

AFAIK, there are no voice activated PTTs. The problem is, how would the system distinguish between inter-crew comms as those meant for the facility you're working?   A button is also a very positive thing.  It's hard to get it wrong.  I did once fly as talking ballast in a small helicopter which although it had  PTTs, both milkes were live when either was pressed.  I found that out when I swore quite loudly as a plank driver just missed us in the circuit, as the PF called "downwind".  It was a "tea and no biscuits" chat with the ATCO when we landed :)

Im not sure of any VOX systems. The only one I can think of is the VOX system on the Apollo rocket that was installed due to pilot work load.

@Dave See here.

The headsets worn by each flight crew member typically feed into an integrated comms/cockpit voice recorder system, where whoever is wearing a headset (and the CVR) will always hear:

anyone else wearing a headset,
the frequency(cies) being monitored on the comm stack, and
the flight attendant handsets (if active and patched to the flight deck).

There is then a PTT button for each headset (usually on the yoke for pilots, back when there was a flight engineer his was a button on his desk or clipped to his uniform) allowing transmission on the selected COM frequency. In simpler systems the entire "cockpit loop" is transmitted, while more complex radio systems provide degrees of isolation so other members of the flight crew can talk while the radio is open.
As far as who talks to ATC, that's usually worked out between the members of the flight crew, so it can vary between flight crews and also depends on the phase of flight, which one is the pilot flying etc. In some crews the pilot flying is also the pilot talking, while in other crews the pilot not flying (if not on "in-seat rest") will handle secondary tasks including navigation and radio.

On Boeing airplanes, flight duties are divided in PF (Pilot Flying) and PM (Pilot Monitoring). This duties are defined at the beginning of the flight because it determine the "area of responsibility" for each pilot during all the flight or for phases of flight (for example: Captain can be PF during taxi and PM during the rest of the phases of flight).
In general, the area of responsibility for the PM, include the duty of COMMUNICATIONS, so this pilot is the one responsible for talking on the radio.
During the cockpit preparation the Captain determine which pilot will be the PF and the PM for the flight (or for the each phase of flight), so in that way every crewmember knows what are their duties, and who is going to be in charge of the radio.
I don't know the exact terms for PF and PM for other type of aircraft (I think that for AIRBUS is Pilot Flying and Pilot Not Flying) but I think this flight duty division works for all types...
Hope this helps...

Many airplanes, large & small, have 2 (or more) radios that can be used individually via intercom settings. Thus the pilot may be talking to or monitoring Center/Approach/Tower, while the Copilot monitors ATIS/AWOS/ASOS and gathering the weather, altimeter, runway in use, or talking to the FBO for parking arrangements after landing, or talking to Company for parking gate assignment/changes, etc.  So it's not strictly one pilot on one radio only.


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Does a Mach 1 plane hear a sonic boom from a Mach 3 plane overtaking him?
Let's say Bob is flying at Mach 1 and then John, in another aircraft, overtakes him at Mach 3. They are flying parallel to each other, say 500 meters apart. And there is no wind. Their relative speed is Mach 2 of course. Does Bob hear a sonic boom from John, and why or why not?
Please explain it sufficiently enough so that I can apply it to other situations. For example, would it still work if their speeds were more like Mach 1 and Mach 1.25?

To answer the question:
Yes, and even more, both would be able to notice the sonic boom of the other aircraft.
Why
To understand why, you have to understand how sonic booms are created.
Sound is nothing more than waves of different air pressure travelling through the air. Once released by an aircraft or other object, they start to propagate in all directions with the speed of sound in the resting air. (This means the waves do not travel faster than sound, even when the aircraft does!) If we mark e.g. the regions of high pressure, we get growing circles, the center of each being the point where it was created by the aircraft.
If the aircraft is moving, the circles will not be concentric, like in this animation for an object traveling at half the speed of sound:

If the aircraft is moving with the speed of sound, the circles start to touch each other on a line perpendicular to the track of the aircraft, just where the aircraft is. This is not so clear on the next animation, as there are simply not (big) enough circles.
However, on this line, all the regions of high pressure sum up to a region of 'very high' pressure. This is the shock wave, which is often referred to as sound barrier. (The term also describes just the insurmountable speed limit encountered by non-supersonic aircraft.) If this shock wave passes an observer on the ground, he will hear a sonic boom.

As the aircraft accelerates more, the shock wave isn't perpendicular any more, instead, you get two lines. In reality (3D), you get a cone with the aircraft at the tip, and the opening angle is defined by the speed of the aircraft.

Now, to answer your question: As the tip of the cone is where the aircraft is, this cone travels with the aircraft, and with its speed.
So, if an aircraft is overtaken by a supersonic aircraft, it will be overtaken by the cone-shaped shock wave of that aircraft a little bit later, too. Also, the faster aircraft will pass the shock wave of the slower aircraft.
Remarks

An aircraft itself is very loud, but also dampens every noise from outside, and headsets and helmets may contribute to the dampening, too. So, I don't know if a pilot would hear something from the boom. Maybe he does not, but the aircraft definitely crosses the shock wave.
My explanation does not describe where the sound comes from. While the sound of a subsonic aircraft mostly comes from the engines, it comes from the compression of air at the surfaces of supersonic aircraft.
This cone sometimes is visible, as on the following picture of an F18, stolen from air-attack.com. This is because the high pressure region is followed by a low pressure region, where air expands and moisture may condensate. The picture also shows that there are usually two shock waves, each where the cross section of the aircraft changes significantly.  One is generated near the canopy and one near the wings. EDIT: As commented by @PeterKämpf, this picture does NOT show an F18 at supersonic speed. It is at transsonic speed, which is close to, but still below mach 1. The fog forms due to other effects.

However, here is an other picture of a Northrop T-38 at mach 1.1, showing the shock waves, from wiki commons:

Nice explanation, but the F-18 picture doesn't quite fit in, because the plane is flying close to, but still slower than Mach 1. The picture shows a condensation cone which is only visible below Mach 1.

Also planes cannot travel at Mach 1 they must punch through it. If they stay at Mach 1 the pressure wave keeps building until it rips the plane apart.

Both planes would create a sonic boom.  The boom is a pressure wave.  The pressure wave is not aware of the relative speed of the two planes.  The slower plane would hear the boom when then cone overtakes it.
The more curious question is will the faster plane actually hear the boom from the slower as it approaches.   I think the answer there there is the faster plane will hear the sonic boom of the slower when it crossed the cone - it just starts in the cone.
Sonic_boom
I just have one year of freshman physics so don't take this as authoritative.
I spoke with a fighter pilot (and instructor) and he said the trailing plane would feel the shock wave going in a out but not that big a deal.  Mainly the air pitot tube speed indicator would go off for a bit.  Since you are moving with the percussion it speeds out the force.  The individual cone when they meet would have minimal interaction as relative to each other they are subsonic.

This should be asked in the physics part of stack exchange, but here are some thought about sonic boom:

To answer your question, you must be clear about frame of reference (this is one reason why you should ask your question in physics.SE)
A sonic boom is created because an aircraft is going faster than the speed of sound. This speed is measured relative to the airmass the aircraft is flying in.
A sonic boom is heard only in a specific place relative to the supersonic aircraft. The boom is due to the shock wave genereted by the aircraft.
In the case you ask for, if any traveller cross the shock wave of the slower supersonic aircraft (whatever the direction), he can hear a sonic boom. This sentence is true as long as you consider a supersonic aircraft (either the slower or the faster)
Any sound is less strong when you are far away from the source of this sound. This include shock wave (and that's why you won't hear a supersonic aircraft flying few thousands of miles away from you)

How is the frame of reference relevant? We're not talking about anything even close to the speed of light, so Newtonian relativity is a near-perfect approximation. And, in any case, the frame of reference is explicitly stated in the question, since it asks about the experiences of the two pilots.

@DavidRicherby My answer don't use anything else than Newtonian relativity. Speed cannot be defined without speaking of frame of reference. The question shows some confusion about the definition of speed of sound. I think it is important to specify that speed of sound is relevant only relative to the frame of reference of the airmass.


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What makes the loss of a flap track fairing strongly increase fuel consumption on Airbus planes?
Inspired from this question about the loss of a flap track fairing.
In the answer by Thunderstrike, a part of the configuration deviation list of an Airbus A320 is linked. In it, it says:

One fairing may be partly or completely missing.
Performance

Fuel consumption is increased by 3.14%

That really is a lot!
I found a post on Airliners.net citing the CDL of a B757:

Straight out of the MEL/CDL manual for the 757, The item falls under CDL 27-51-1 which states there are 6 installed [Flap Track Fairings, Remark] on the aircraft and up to one per side may be missing. No fuel burn penalty but there are a few small weight penalties, no speed penalty. Our A-300's with the same CDL item does have a 1% fuel burn penalty and only one can be missing out of the 10 installed. I see this item come up 3 or 4 times a year, no biggie.

What causes this very high increase in fuel consumption on Airbus planes?
What are the differences between the fairings of a Boeing and an Airbus (because Boeing has no increased fuel consumption, Airbus has)?

I'd say that a weight penalty could be reinterpreted as a fuel penalty (i.e., the reduced payload translates in a shorter range, if you ignore the lower payload limitation), so I don't really think it is a special "feature" of Airbus.

and the reason why they allow for such fuel conservation is explained here: https://aviation.stackexchange.com/q/7955/1467

I used to compete in aircraft recognition and one of the obvious differences comparing the two aircraft is the prominence of flap track fairings.  At a distance it was always easy to ID an Airbus because of large and many flap track fairings compared to Boeing.  Interesting.

The fairings double as anti-shock bodies (or Küchemann carrots) and help to reduce shocks on the lower side of the wing. Below you see a wind tunnel test from the Vickers VC10 development where flow visualization using oil was used at Mach 0.9:

Wing without (left) and with (right) anti-shock bodies at Mach 0.9 (picture source). The light line on the left side indicates a strong shock which increases drag substantially over the situation pictured on the right where the flap track fairings have been used to smooth out the sudden reduction in cross sectional area from the rear-loaded wing airfoil.
Please read this answer if you are unsure why aircraft experience more drag as they approach the speed of sound. By stretching the aircraft body over a larger length, the gradients in the cross sectional area distribution can be made more shallow such that the air has more opportunity to make way for the aircraft.

In image A) there is a bright, sharp, narrow white line about half-way along the wing. This, as I understand it is the shock line. In image B) there are equally bright (to my eye), but very broad/spread out white areas. AIUI, the fact that they're spread out, and not in a thin, crisp line is what makes them not shock lines and therefore significantly lower contributors to drag. Is this a correct assumption?

@FreeMan: The more interesting lines in the left picture are the very thin, parallel ones which show that past the shock line flow travels only towards the wing tip and slightly forward. This looks like fully separated flow to me. The right side picture does also show shocks, but they are weaker and do not cause separation.

Thanks for the interpretation. For those of us not versed in the black arts of aerodynamics, that's quite helpful! ;)


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How do pilots clear an aircraft to fly through a hurricane?
This video shows a pilot (Tim Gallagher/NOAA) flying what looks like a small jet into the eye of hurricane Matthew by Miami. It's clear that it is quite an interesting ride. The (small) caption states that the purpose of the trip is for "scientific research". I naïvely imagine that the research in question is the gathering of an idea of the velocity field, pressure, atmospheric aerosols, humidity, and spectral density/radiative transfer rates between different "parcels" of air within the large and complex structure in three dimensions. I'm a physicist, but not an atmospheric physicist, so I'm not entirely sure why you would undertake such an expensive and potentially dangerous trip like this.
As a result, I've got two questions:

How exactly would you pick an aircraft to survive a trip through the eye of the storm. Is it a big risk? I imagine that $V_{ne}$ of a fighter jet is an order of magnitude bigger than the bulk flow of any storm on earth, but the aircraft that he is in is not a fighter jet, and I could readily believe that there could be turbulent patches of air moving very quickly. Likewise, what is the risk of stalling the entire wing, or unexpectedly entering chaotic regions of airflow that could cause problems to control surfaces? How would you perform a "risk assessment" on what I presume is -- by the fact that it is a scientific flight -- the unknown?
What exactly are aircraft storm chasers after that cannot be measured from elsewhere? Satellites would provide potentially quite a lot of information about the highest few layers of the structure, and one could imagine that overflying it and dropping probes would be much safer, and more readily parallelised to getting some sort of information from more than exactly one path in $(\mathbf{r},\,t)$.

Here's a screenshot from the video, showing the sort of large wind/water flow the aircraft is experiencing; it's quite dramatic!

This seems to overlap a lot with this question. The second part of your question about what can only be measured directly inside a storm might get a better response on earthscience.SE, but maybe someone here will know.

Your title does not match the questions in the body. Do you want to know how a pilot gets clearance for the flight?

The aircraft they're flying is an Lockheed WP-3D Orion, not a jet- the NOAA doesn't fly its Gulfstream IVs into the storm, as I've noted elsewhere.
There is always a risk associated with piloting an aircraft through weather that pilots are usually told to avoid. But they have been doing it for decades, so, with proper training and planning, it appears that it's not that risky. The top speed of the WP-3D is listed as over 750 kmph. The $V_{ne}$ would definitely be over that. The top wind speeds in Category 5 hurricanes (the highest in the Saffir-Simpson scale, of which Matthew is one) are over 250 kph, though most hurricanes aren't quite that strong.
The NOAA seems to be aware of this issue; in this reddit AMA, they agree that wind shear could damage the aircraft and lead to loss of control:

Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control.

However, with proper training and by avoiding the most dangerous areas using real-time data, they can fly the aircraft within the safe limits.

NOAA pilots and crew routinely fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for "hot spots" of severe weather and shear that they can often identify on radar and avoid if it's too severe.

The aircrew has things to worry about other than $V_{ne}$- the high velocity winds in the up and downdrafts can result in high 'g's, while the wing is susceptible to flutter in these conditions. This blog chronicles cases where the mission was abandoned due to aerodynamic flutter and a case where nearly 5.7 'g's were experienced. Another blog describes the penetration of Hurricane Hugo, that resulted in a near fatal disaster and grounding of the aircraft. From hurricanescience.org

As they flew through the storm’s ey(e)wall they encountered sustained winds of 297.7 km/h (185 mph) (gusting to 315.4 km/h [196 mph]) and extreme updrafts and downdrafts (32-72 km/h [20-45 mph]). The massive jolts rocked the aircraft, causing gear such as tools, computers, all items in the galley, and even the 200 lb life raft to be dangerously thrown about the aircraft’s interior. The cockpit gravitational force(G)-meter, became pegged at 5.6 G's and -3.9 G's . Upon reaching Hugo’s eye, one of the engines exploded and had to be shut down. In the calm of the hurricane’s eye, the plane made repeated circles, a risky maneuver with the loss of one engine. NOAA 42 was able to successfully exit the hurricane ...

The crews also take some safety measures, such as flying at higher altitudes (UAVs are used at lower altitudes) to reduce the associated risk. As every hurricane is different, risk assessment is difficult and the crew has to fly sometimes based on imperfect information. From hurricanescience.org:

Flying into the eye of a mature hurricane presents significant hazards as strong updrafts in the hurricane’s eyewall can give way to equally strong downdrafts on the measure of a few hundred feet. Horizontal wind speeds can also change by more than 161 km/h (100 mph) in a few seconds of flight time as the plane moves through the eyewall, putting strains on the airplane and its crew. Because of these hazards, Hurricane Hunter flights are typically done at 10,000 ft (3,048m).

There are two types of flights these aircraft are used for- reconnaissance and research. While satellites may give quite a bit of data, there are some things they can't provide and can be obtained only by flying into the storm. The main purpose of reconnaissance flights is to track the center of circulation and gather data, which will help determine the threat posed by the hurricane and the path it will take. NOAA notes:

The primary purpose of reconnaissance is to track the center of circulation, ... , and to measure the maximum winds. But the crews are also evaluating the storm's size, structure, and development ... . Most of this data, which is critical in determining the hurricane's threat, cannot be obtained from satellite.

The research flights are for varied purposes such as to gather data about the structure of the storm and its interaction with environment, which will help them predict future hurricanes better. In fact, in the above mentioned flight into hurricane Hugo, the crew observed a long suspected phenomenon for the first time, which would not have been possible using satellites:

Later analysis of the data taken during our amazing flight into Hugo revealed that we hit a tornado-like vortex embedded in the eyewall when the hurricane was at its peak intensity. These eyewall vortices had been suspected but never before observed, and ongoing research suggests that similar vortices may be responsible for some of the incredible damage hurricanes can inflict when they strike land.

While it is true that it is possible to drop probes while overflying hurricanes, it is not always possible- some of the larger ones have cloud tops reaching over 50,000 ft, higher than the ceiling of aircraft available (for example, the cloud tops of Hurricane Matthew reach over 16km), making it quite difficult to drop dropsondes and the data is relayed to the aircraft which then transmits to the center using satellite.
In recent times, UAVs are being used increasingly for this purpose (NASA uses a Global Hawk for this purpose) and, maybe, in the future, they'll replace the manned aircraft altogether.

Well, the WB-57 can get over the top of such a hurricane, but that's a rather specialized plane ;)

Additional research items can be gleaned from comparing the data from the aircraft transit to a coordinated satellite pass. This was they can marry the data sets to improve the interpretation of the the space based image. Weather spotters help regular NOAA offices with this process, time correlated on-the-ground damage assessment can be compared to imagery to improve severe weather alerting for future events. The reason why Jets are not usually used is because, it is harder to get as many sample points. By flying slower they reduce the traveling salesman problem around sample frequency.

Anyone in the US who is interested can check out https://www.weather.gov/SKYWARN and look at your local weather office which is usually located at a Class A, B or C airport. Spotters are especially needed in areas close to between Weather offices because radar can't usually see below 3000' AGL due to ground obstructions and curvature of the earth issues. This was a big loss NOAA had when they upgraded long range radar capabilities and closed offices. there is now very little Doppler overlap between offices.


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Why don't we get more user-friendly weather reports than METARs?
This question is not about how to decode METARs or what software or tools can be used to decode METARs. It is about why we are still using METARs today.

Given that we have smartphones with video messaging capability, cloud services with machine learning today, I struggle to understand why we are still reading METARs:

It is difficult to read. Sure it is not hard to learn, but the fact that you have to learn it means it's not intuitive
It is designed at an age when text are transmitted by telegraph and every character counts
It is trivial to program a parser and render more friendly messages

For me I always feel that METARs are meant for machines; there has got to be something better for pilots which happens to be human. Instead of having this on my flight plan:

KSFO 111456Z 16015KT 9SM RA FEW012 SCT030 OVC070...

Why not just:

Station: KSFO
Time: Mar 11 14:56 UTC
Wind: 160 degrees, 15 knots
Visibility: 9 statue miles
Weather condition: rain
Clouds: few at 1,200 feet; scattered at 3,000 feet; overcast at 7,000
  feet
...

The problem is not about decoding them. Most pilots can read them, and by the time a new pilot completes his PPL training he should have no trouble reading either. The point is everybody around the world is reading them, today, in the 21st century!
I don't know if I'm the only one, but processing the METAR string during flight prep takes a couple of instruction cycles away from my brain (and I have a tendency to check whether the software got it right if I happen to be using one, so I will be processing it anyway, which is a good habit I suppose). I am sure some of that brain power can be better spent somewhere else: reading section charts, route planning, picking an alternate, etc. Scaling this to the global scale? It means every pilot is paying more attention to other things, as opposed to reading coded weather strings which really is the job of a machine. The result? Better flight safely.
So, why haven't we began the move?

it's a hole in the market, a METAR parsing app.

Its a hold-over from its introduction in 1968 as a way to transmit a lot of information in a compact, semi-readable form. There are a lot of apps/websites that automatically translate the METAR/TAF information for you, even convert from UTC to local.

Because aviation regulations move at the speed of government...

everybody around the world is reading them, today does not sound like a compelling reason to change something designed for people who understand them.  Do you suggest that every industry should talk in a way that "everybody around the world" can understand?

Personally, I find coded METARs far easier to read and understand than de-coded ones. I can scan the code faster, it is standardized, and I find it easier to parse. Seeing SCT008 tells me exactly what I need to know faster than scanning through a column of decoded text to find Clouds: Scattered at 800 feet above field elevation, or whatever the decoder might choose to parse the information as. When given the option of having a coded or a decoded METAR, which is a widely available option, I prefer the coded version.

Also good to say: METAR has a standardized format. That means it is machine readable. Computers can understand what is going on at airports. Very nice thing. And the other one: If it would be pre-formatted, you could only have this format. With the standardized format, you can easily format it how you want it.

If it ain't broke...
For what its worth many new aviation apps like ForeFlight and the such display the information in user friendly formats.

(source)
To be honest I have not decoded a METAR in some time. You are correct that the system comes from a time when transmission bandwidth was not what it is today. Keep in mind things in aviation are slow to change and many other nations also report weather in the METAR format so its useful to know how to read one if you fly abroad where they don't have the kind of infrastructure and applications we have available here in the US. Since official bodies like NOAA and the FAA broadcast METARS its far easier for a company like ForeFlight to parse and display what they can already get than try to make the government change the format.
I would love to see a broadcast of the METAR data in easy to read JSON format....
There are some packages out there for dealing with them in software if you need to.
There is also something to be said for a format that can easily be transmitted over various communication mediums. While morse code is not that popular anymore and most things are broadcast digitally the METAR format would still allow the information to be transmitted as is, in the event one of the more modern systems failed.
Since the question has been edited a bit ill expand my answer to reflect. There really is no reason to change the format since most places you get the METAR from decode it for you anyway these days. While it has been pointed out there are some quick things that can be determined from reading them in their current format there is no information that is planed to be added to them so there is no reason to change (or expand) the format. A great deal of infrastructure exists around producing and decoding them as is, which would be a large task to overhaul and in the end produce nothing different than what exists now.

One use case where the compact METAR format is tough to beat is rapidly scanning meteorological trends. For that purpose, METARs are user friendly in the sense that they are expert friendly.
For example, a friend invited me to fly with him today on an Angel Flight mission that would have required a landing at Nashville. We had to scrub because the weather is unfavorable and not getting any better according to the TAF and ADDS METAR Data:
Data at: 1616 UTC 11 Mar 2016
KBNA 111553Z 36006KT 1SM R02L/4000V6000FT -DZ BR OVC005 14/12 A3024 RMK AO2 DZB51 SLP239 P0000 T01390122 $
KBNA 111541Z 35005KT 1SM BR OVC005 14/12 A3024 RMK AO2 SFC VIS 1 1/2 $
KBNA 111527Z 01006KT 2SM BR OVC005 14/12 A3022 RMK AO2 SFC VIS 2 1/2 $
KBNA 111453Z 02008KT 3SM BR OVC005 14/12 A3021 RMK AO2 SLP227 T01390122 53015 $
KBNA 111451Z 02007KT 3SM BR OVC005 14/12 A3021 RMK AO2 $
KBNA 111434Z 01007KT 2SM BR OVC004 14/12 A3020 RMK AO2 $
KBNA 111353Z 02007KT 5SM BR OVC004 14/12 A3019 RMK AO2 SLP219 T01390122 $
A subsecond scan confirms that the low overcast ceiling is stuck at four to five hundred feet AGL for the past several hours. Try outdoing that using the same information in decoded METAR format.

It also says "$ FIXME . . . . $ FXME . . . . $ Yeah guys it's cool, just ignore my maintenance requests . . . . $ You'll regret this later when I say sky clear winds calm and it's actually a hurricane . . . . " :-)

Because the METAR format is standardized and a large number of systems know how to read and process them. Lots of applications consume METAR data, from weather websites to ACARS terminals in cockpits to pilots' brains. An application can read in METAR data from many stations and use it to produce, say, a map of winds across a region or a graph of temperatures over time.
This all works because everyone has a common understanding of the format, and there are a number of code libraries for major programming languages that assist in generating and parsing the METAR format. If you change the format to be human readable, all of that breaks. As with many popular standards, changing it would require the immediate global cooperation of everyone at once.
It makes far more sense to keep the METAR format as it is (or to extend it with new elements to accommodate new reporting needs) and build tools to present the information in user friendly ways, as ForeFlight and many others do.

Other answers have addressed some of the reasons why the METAR transmission format should stick around. I'll add that there is a lot of machinery in place that would be expensive to replace in order to make such a change, so it wouldn't happen even if we wanted it to.
If something was going to change, it could be to eliminate the testing requirement that all new pilots are able to read and interpret METARs. As has been pointed out, there are many applications both for devices and on the web that can translate them for you, including aviationweather.gov. So it's dubious reasoning to assert that a person cannot be a competent pilot unless they can interpret a METAR without assistance.
Useful skills should be trained, but only skills and knowledge required to be competent should be tested.


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What does scheduled departure time exactly mean?
I am trying to understand the (sometimes subtle) differences between various terminologies concerning flight schedules. What I find difficult to understand is how exactly the scheduled departure time is defined. My initial thought was that it refers to the time where the aircraft leaves the gate, after the boarding of passengers. However, I noticed that this definition is closer to the term "Target Off - Block Time".
ICAO ATFM TERMINOLOGY AND COMMUNICATIONS, page 7:

Target Off - Block Time:
The time that an aircraft Operator or Ground
handler estimates that an aircraft will be
ready to startup/pushback immediately upon
reception of clearance from the tower.

Could someone clarify this issue?

"Scheduled departure time" is not a term used in the operational environment. It is a term you will find on the passenger side of things, and means the time at which the aircraft is scheduled to leave the departure gate.
There are several different operational terms related to departure time:

ETD, Estimated time of departure. The estimated time at which an aircraft will become airborne.
SOBT, Scheduled off block time. The time that an aircraft is scheduled to depart from its parking position. More or less identical to "scheduled time of departure".
EOBT, Estimated off block time. The estimated time at which the aircraft will commence movement associated with departure.
TOBT, Target off block time. The time that an aircraft operator / handling agent estimates that an aircraft will be ready, all doors closed, boarding bridge removed, push back vehicle present, ready to start up / push back immediately upon reception of clearance from the TWR.
AOBT, Actual off block time. The time the aircraft pushes back / vacates the parking position.
TSAT, Target start up approval time. The time provided by ATC taking into account TOBT, CTOT and/or the traffic situation that an aircraft can expect to receive start up / push back approval.
ETOT, Estimated takeoff time. Forecast of time when aircraft will become airborne taking into account the EOBT plus EXOT.
EXOT, Estimated taxi-out time. The estimated time between off-block and take off. This estimate includes any delay buffer time at the holding point or remote de-icing prior to take off.
TTOT, Target takeoff time. The Target Take Off Time taking into account the TOBT/TSAT plus the EXOT. Each TTOT on one runway is separated from other TTOT or TLDT to represent vortex and / or SID separation between aircraft.
CTOT, Calculated takeoff time.A time calculated and issued by the Central Flow Management unit, as a result of tactical slot allocation, at which a flight is expected to become airborne.
ATOT, Actual takeoff time. The time that an aircraft takes off from the runway.
ATD, Actual time of departure. Used either to have a meaning identical to ATOT, or AOBT, depending on context.

I've probably missed some, too.

You've given me an acronym overload!! Must jump off roof!!!

What is the difference between the estimated and target times? Your description for TOBT also says estimate and then the further target times are based off that.

@JanHudec estimated times are based on the filed flightplan, target times are based on a number of other factors, including flow regulations. And I think the target values, at least TOBT and TSAT, have lower tolerances than EOBT (which must be adhered to +/-15 minutes)

They are both the same.  That definition "ready to start/pushback" in the block quote is pretty much the same as leaving the gate, since leaving the gate is the pushback.

Thank you John for your response. So scheduled departure time indeed refers to the time that the aircraft leaves the gate, correct?

Yes.  In recording delays and cancellations for statistical dispatch reliability analysis, the on-time departure clock starts when the doors close and pushback starts.  On-time is considered to be within 15 minutes of the scheduled time.